Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and/or improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air may be received at a compressor inlet location. Hydrocarbons ingested from purge air, positive crankcase ventilation (PCV) and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, and the accuracy of the sensor, and thus, measurement and/or control of EGR, may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: during boosted operation, adjusting exhaust gas recirculation (EGR) based on a first output of a first oxygen sensor positioned in an intake passage and exposed to EGR gases and a second output of a second oxygen sensor not exposed to EGR gases and exposed to positive crankcase ventilation and purge flow gases. In this way, the effect of purge and PCV hydrocarbons on an intake oxygen sensor output may be accounted for and used to determine a more accurate EGR estimate using two intake oxygen sensors.
For example, the first oxygen sensor may be positioned in an intake passage of the engine, downstream of where an EGR passage couples to the intake passage. The second oxygen sensor may be positioned in the intake passage upstream of where the EGR passage couples to the intake passage and downstream of where positive crankcase ventilation (PCV) and purge flow hydrocarbons enter the intake passage during boosted engine operation. As such, the first intake oxygen sensor may be exposed to both EGR flow and purge and PCV flow hydrocarbons and the second oxygen sensor may be exposed to just purge and PCV flow hydrocarbons when the engine is boosted. EGR may then be estimated based on a difference between a first output of the first oxygen sensor and a second output of the second oxygen sensor. A controller may then adjust EGR based on the estimated EGR. By taking a difference of the two oxygen sensor outputs, the effect of purge and PCV hydrocarbons on the first output may be removed, thereby increasing an accuracy of the EGR estimate and the resulting EGR control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.